The present invention relates to a dielectric film having an ABO3 perovskite structure with a high dielectric constant suitably used as a dielectric film for sandwiching upper and lower conducting electrodes in a thin film capacitor and a method of fabricating the dielectric film.
Recently, electronic equipment is desired to have small size and thickness. Electronic equipment includes a circuit substrate or the like mounting electronic components (elements) such as an IC and a capacitor, and therefore, an electronic component and a circuit substrate used in electronic equipment are also required to have small area and thickness. Among such electronic components (elements), a capacitor in particular occupies a large area, and hence, reduction in its area can lead to a great effect in reducing the area and the thickness of the electronic component.
On the other hand, a memory utilizing charge and polarization of a capacitor is remarkably widely used these days, and various structures have been proposed for a capacitor. For example, in order to realize a compact and thin capacitor, not only a semiconductor substrate is used but also a compact, light and inexpensive substrate, such as glass, ceramics, a metallic foil and a flexible organic polymer film, is used, so as to provide a compact and light thin film capacitor having a laminated structure including electrode thin films and a highly dielectric thin film.
In particular, a dielectric film having a perovskite structure whose stoichiometric composition is represented by ABO3 (wherein A and B are metal atoms) has a large dielectric constant and is a significant material in reducing the area and the size of the electronic component. Also, a dielectric film having the perovskite structure has, depending upon its orientation and kind, a hysteresis characteristic that predetermined polarization remains after removal of voltage application, and hence, such a dielectric film can function as a ferroelectric film. Since SrTiO3 (namely, so-called STO), that is, one of dielectrics having the perovskite structure, has a comparatively large dielectric constant, its use in a thin film capacitor for a non-contact IC card is now increasing.
A dielectric film having the perovskite structure is generally formed by CVD such as so-called MOCVD utilizing a reaction of an organic metal, and is sometimes formed by sputtering, deposition or the like for the purpose of improving through-put and reducing fabrication cost.
Although a dielectric film having the perovskite structure has the aforementioned good characteristics but also has unsolved problems as follows:
First, although a dielectric film having the perovskite structure has a large dielectric constant and a ferroelectric property, it is known that these dielectric characteristics depend upon the crystallinity of the dielectric film. It is known, for example, that the dielectric constant and residual polarization can be lowered when the dielectric film is poor at the crystallinity and the orientation. When the sputtering is employed for the purpose of fabricating an inexpensive thin film capacitor, the composition of the dielectric film is often shifted from the stoichiometric composition, and in such a case, the crystallinity is degraded. As a result, the desired dielectric characteristics cannot be always attained.
Also, when the thickness of the dielectric film is reduced so as to reduce the thickness of the element and increase the capacitance, a leakage current tends to unavoidably increase. In particular, when a dielectric film having the ABO3 perovskite structure includes a large number of crystal defects, much leakage occurs. Such crystal defects include loss of an oxygen atom (O) (oxygen loss) in a crystal lattice of the perovskite structure represented by ABO3 and shift of the existing ratio between the metal atoms A and B from 1:1. These crystal defects can result in various problems, and for example, breakdown can easily occur in the dielectric film, data stored in the dielectric film serving as a memory can be easily volatized, or when the dielectric film is used as a ferroelectric film, sufficient polarization cannot be attained by applying a voltage.
These problems are particularly conspicuous when the dielectric film is formed by sputtering or deposition carried out at a low temperature. In other words, when the dielectric film is formed at a high deposition rate, in particular, at a low temperature by the sputtering or deposition so as to improve the through-put for the purpose of lowering the fabrication cost, a phenomenon such as degradation in the crystallinity and occurrence of crystal defects can be easily caused. In fabrication of a thin film capacitor formed on a flexible organic film with low heat resistance as in application to an IC card or the like, the dielectric film is unavoidably formed at a low temperature. Accordingly, it is difficult to fabricate, at low cost, a thin film capacitor including a dielectric film with the ABO3 perovskite structure, such as an STO film, formed on a substrate with low heat resistance.
An object of the invention is providing a dielectric film having the ABO3 perovskite structure that can be fabricated at low cost with keeping good dielectric characteristics such as a leakage characteristic, and a method of fabricating the dielectric film.
The dielectric film of this invention has a perovskite structure with a stoichiometric composition represented by ABO3, including a group II element A, a group IV element B and oxygen O, and the perovskite structure includes nitrogen.
Accordingly, owing to nitrogen included in the perovskite structure, leakage caused in applying a voltage to the dielectric film upward or downward can be reduced. Also, the dielectric film can attain improved crystallinity and minuteness. Therefore, other characteristics, such as a dielectric constant, residual polarization in applying a voltage to the dielectric film upward or downward, and dielectric loss, can be improved.
In the dielectric film, the nitrogen is bonded to at least one of the element A and the element B. Therefore, an effect to suppress electron tunneling can be definitely attained.
In the dielectric film, the nitrogen is included in the perovskite structure in a concentration of 5at % or less. Therefore, a problem caused by excessive nitrogen can be avoided.
In the dielectric film, the element A is at least one material selected from the group consisting of Sr, Ba, Bi, La and Pb and the element B is at least one material selected from the group consisting of Ti, Ca, Nb, and Zr. Therefore, the invention is applicable to a widely used dielectric film having a perovskite structure.
The dielectric film of this invention may function as a capacitor dielectric film of a capacitor disposed on a substrate made from an organic material, namely, a capacitor formed on a so-called flexible substrate. Thus, a capacitor dielectric film made from a dielectric film with a high dielectric constant can be realized at low cost. In other words, a high quality and highly reliable thin film capacitor with large capacitance, small loss, a small leakage current and a high breakdown voltage can be obtained.
The first method of this invention of fabricating a dielectric film having a perovskite structure with a stoichiometric composition represented by ABO3, including a group II element A, a group IV element B and oxygen O, comprises a step (a) of supplying a gas including an element lighter than Ar and a gas including oxygen into a chamber; and a step (b) of depositing the dielectric film having the perovskite structure including the element A, the element B and oxygen on an object surface by drawing atoms of the element A and atoms of the element B from a metal material into a space within the chamber.
In this method, the existing ratio between the atoms of the element A and the atoms of the element B drawn into the space within the chamber can be adjusted, even at a low temperature, so as to attain a condition for forming a dielectric film having a composition close to the stoichiometric composition of the ABO3 perovskite structure. Accordingly, a dielectric film good at dielectric characteristics can be formed on a substrate with low heat resistance.
In the first method, an Ar gas may be supplied into the chamber in addition to the gas including an element lighter than Ar in the step (a). In this manner, the existing ratio between the atoms of the element A and the atoms of the element B drawn into the space within the chamber can be more easily adjusted to fall within an appropriate range also by utilizing comparatively heavy Ar atoms.
It is experimentally confirmed that a mixed gas including an Ar gas, an oxygen gas and a nitrogen-containing gas is preferably supplied into the chamber in the step (a).
When the atoms of the element A and the element B are drawn into the space within the chamber by sputtering from the metal material in the step (b), the deposition rate for the dielectric film can be increased, which can make the method suitable to mass-production.
When the dielectric film is deposited at a rate of 10 nm/min. or more in the step (b), the productivity is improved, resulting in forming the dielectric film at low cost.
When the object surface used in the step (b) is a top surface of a metal electrode disposed on a substrate made from an organic material, a dielectric film with good characteristics can be formed on a flexible substrate or the like at a low temperature. As a result, an inexpensive capacitor can be fabricated by using the dielectric film of this invention. In other words, the method is applicable to fabrication of a high quality and highly reliable thin film capacitor with large capacitance, small loss, a small leakage current and a high breakdown voltage.
The second method of this invention of fabricating a dielectric film having a perovskite structure with a stoichiometric composition represented by ABO3, including a group II element A, a group IV element B and oxygen O, comprises a step (a) of supplying at least two kinds of sputtering gases having different weights and an oxygen-containing gas into a chamber; a step (b) of setting the two kinds of sputtering gases and the oxygen-containing gas to a plasma state; and a step (c) of depositing the dielectric film having the perovskite structure including the element A, the element B and oxygen on an object surface by striking a target material with atoms of the two kinds of sputtering gases in a plasma state so as to draw atoms of the element A and atoms of the element B into a space within the chamber.
In this method, the elements A and B are sputtered by the at least two kinds of gases having different weights, and hence, a ratio in the sputtered amount between the elements A and B can be varied by varying the flow ratio between the two kinds of gases. Specifically, by adjusting the flow ratio of the at least two kinds of gases, the existing ratios between the elements A and B can be adjusted to be optimal for attaining the stoichiometric composition.
In the second method, the two kinds of sputtering gases are preferably inert gases.